The present invention relates to an electrically multidivided photodetector used for an optical encoder.
Optical encoders have been used in a wide variety of fields for controlling the rotational speed, rotational direction, rotational position and so on of a motor or the like and controlling the travel speed, movement direction, movement position and so on of a moving object. For example, reference should be made to JP 59-40258 A and JP 61-292016 A.
A disk called a code disk is used for the rotational control. A thin plate called a scale slit is used for the movement control. The code disk is provided with optically transparent slits arranged at prescribed intervals along its circumference, where optically transparent areas and opaque areas exist alternately along the circumference of the code disk. On the other hand, the scale slit is provided with optically transparent slits arranged at prescribed intervals along its one side, where optically transparent areas and opaque areas exist alternately along the above-mentioned side of the scale slit.
For the rotational control, the rotational amount and the rotational speed are detected by counting the number of slits of the rotating code disk by means of a photodetector. For the movement control, likewise, the movement amount and the travel speed are detected by counting the number of slits of the moving scale slit by means of a photodetector. Further, the rotational direction or the movement direction can also be detected by using the two kinds of signals having a phase difference of 90 degree, the signals being formed by the positions of a plurality of photodiodes constituting the photodetector.
In the code disk and the scale slit, the opaque areas is formed by an opaque material such as ink on a board made of a transparent resin or glass, so that a stripe pattern is formed by the opaque areas and the transparent slits between the opaque areas. Otherwise, in some cases, the transparent slit is formed by providing holes through a metal plate.
Regarding the configuration of the photodiodes, as shown in FIGS. 11A and 12A and described in the above-stated patent documents, one set of photodiodes is constructed by four adjoining segmented photodiodes Pd1 to Pd4 which are formed on one semiconductor chip under a electrically separated state. The slit pitch of the scale slit is made 4-fold of the arrangement pitch of the segmented photodiodes. Then, one set of the segmented photodiodes is arranged as shown in FIG. 11A. In some cases, as shown in FIG. 12A plural sets of the segmented photodiodes are arranged in order to level positional variations in the quantity of light from a light source. Then, outputs of the segmented photodiodes Pd1 to Pd4 are respectively connected to outputs of the other corresponding segmented photodiodes as shown in FIG. 12A (refer to FIG. 6B of JP 61-292016 A).
Among the photocurrents outputted from the segmented photodiodes Pd1 to Pd4 when the scale slit moves, a pair of photocurrents A+(Pd1) and A−(Pd3) and a pair of photocurrents B+(Pd2) and B−(Pd4), which have a phase difference of 180° each other as shown in FIGS. 11C and 12C, are inputted to a comparator (not shown) so that the levels of the input signals are compared with each other. Consequently, two digital rectangular waves having a phase difference of 90° are obtained, as indicated by CHA (a channel A) and CHB (a channel B) in FIGS. 11D and 12D.
In the above case, the design value of the resolution of the photodiodes is determined by the arrangement pitch of the segmented photodiodes Pd1 to Pd4. Moreover, the slit pitch (resolution) of the scale slit used for forming two digital rectangular waves, which have a phase difference of 90°, is also limited within a narrow range centered around a theoretical value based on the arrangement pitch of the segmented photodiodes Pd1 to Pd4.
However, the above-stated conventional optical encoder has a disadvantage as follows. That is, the slit pitch of the code disk and the scale slit is made 4-fold of the arrangement pitch of the segmented photodiodes Pd1 to Pd4. Therefore, in the case of a photodetector for the above-stated conventional optical encoder constructed by the segmented photodiodes obtained by electrically quadrisecting one semiconductor chip, it is required to prepare a semiconductor chip such that the value being 4-fold of the arrangement pitch of the segmented photodiodes coincides with the slit pitch (resolution) of the desired code disk or scale slit.
Moreover, the segmented photodiodes Pd1 to Pd4, which constitute the photodetector, are arranged in a line in the above case, and therefore, the slit pitch (resolution) of the code disk or the scale slit eventually obtained becomes theoretically four pitches of the segmented photodiodes.
For the above reasons, the above-stated conventional optical encoder has the disadvantage that the photodetector must be remade according to the slit pitch (resolution) of the code disk or the scale slit every time the required slit pitch (resolution) of the code disk or the scale slit is changed.